Cook and stir onion and garlic in 2 T of oil in a big pot until the onion is tender. Stir in broth and bell pepper, ground chilies, basil, salt, pepper, and tomatoes. Heat to boiling. Reduce heat and simmer uncovered for 30 minutes. Heat oil in skillet and cook tortilla strips until crispy. Divide strips and chicken and avocado in a bowl, add soup, then top with cheese. Serves 4